Rule of Twelfths and
the Rule of Tenths for Quick Tidal Calculations.

The
Rule of Twelfths or if you are more
used to metric, the Rule of Tenths is used to give a rough estimate of
the
state of the tide.

Most Tide
Tables only give
the times and heights
of high and low water, in relation to the chart datum.

Assuming that the tide
in your area has a
normal semi-diurnal cycle, either of these rules of thumb can be used
to
provide a quick estimate of the height and flow of the tide for times
in
between high and low water.

The
Calculation.

First thing you need is the tidal
range that’s the difference between high water and low water for the
day
in question.

So at the end of the first hour of a rising tide, it will have risen
one
twelfth or 10% of the range.

At the end of the second hour it will have risen 1/12 for the first
hour plus
2/12 for the second hour making a total of 3/12.

Using percentages you would have 10% for the first hour plus 15% for
the second
making a total of 25% at the end of the second hour.

In the same way at the end of the 4th hour the tide would be 75% of the
range
higher than low water or 25% lower than high water.

To get the actual depth of the water two hours after low water, that
25% of the
range needs to be added to the height of low water and the height shown
on the
chart (some of these figures might be negative).

Caveat.

I
feel I must reiterate that these rules are just
methods for giving a quick, rough approximation only and should be used
with
great caution when used for navigation.

Tidal streams for instance don't always have a simple relationship with
the
tidal height.

There are many locations where the nature of the sea bed and coastline
cause
all sorts off anomalies.

Some coasts have double high waters and others double low waters and
there can
be abnormal flow rates within the tidal sequence.

And the Tide Tables themselves are predictions based on previous
experience and
the figures for times and heights are subject to modification by wind
and
atmospheric pressure.

So, while rules of thumb, such as the rule of twelfths, allows you to
do quick
mental calculations, navigating at sea should be an accumulation of
input from
as many sources as possible, particularly input from your own eyeballs.

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